1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer or the like which uses an electronic photographing method or an electrostatic recording method.
2. Related Background Art
A conventional image forming apparatus which uses the electronic photographing method or electrostatic recording method, for example, adopts a corona charger (corona discharger) as a charging apparatus which uniformly charges (and discharges) an image bearing body such as a photosensitive body for electronic photographs or an electrostatic recording dielectric body at a required polarity or potential.
The corona charger is a contactless type charging apparatus which is equipped, for example, with a discharging electrode such as a wire electrode and a shield electrode surrounding the discharging electrode to charge a surface of an image bearing body as predetermined by exposing the surface of the image bearing body to a discharging current (corona shower) which is produced by applying a high voltage across the discharging electrode and the shield electrode with a discharging opening set contactlessly opposed to the image bearing body which is to be charged.
In the recent days, many kinds of contact type charging apparatuses have been proposed and put to practical use as apparatuses to charge image bearing bodies and the like to be charged since this kind of charging apparatuses provide an advantage to consume ozone and electric power at lower rates than the corona charger.
The contact type charging apparatus brings an electrically conductive charging member of roller (charging roller) type, fur brush type, magnetic brush type or blade type into contact with a body to be charged such as an image bearing body and charges a surface of the body to be charged at a predetermined polarity and a predetermined potential by applying a predetermined bias to the charging member (contacting charging member or contacting charger hereinafter referred to as a contacting charging member).
In a mechanism of contacting charging (charging mechanism or charging principle), (1) a discharging charging mechanism is mixed with (2) an injection charging mechanism and one of the mechanisms which is governing exhibits its characteristic.
(1) Discharging Charging Mechanism
The discharging charging mechanism is a system which charges a surface of a body to be charged utilizing a discharge phenomenon which takes place in a fine gap between the contacting charging member and the body to be charged.
Since the discharging charging mechanism has definite discharging threshold values for the contacting charging member and the body to be charged, it requires applying a voltage higher than a charging potential to the contacting charging member. Furthermore, the discharging charging mechanism is inevitably accompanied by an adverse influence due to active ions such as ozone since it is based on a principle which is inherently incapable of preventing a product from being formed by discharging though in an amount remarkably smaller than that of a product produced by the corona charger.
(2) Injection Charging Mechanism
The injection charging mechanism is a system which charges the surface of the body to be charged by injecting electric charges from the contacting charging member directly into the body to be charged. This charging is referred to also as direct charging, injection charging or electric charge injection charging.
Describing in more detail, the injection charging mechanism brings a contacting charging member having medium resistance into contact with the body to be charged and injects electric charges directly into the surface of the body to be charged not by way of the discharging phenomenon, or basically not using the discharging phenomenon. Therefore, the injection charging mechanism is capable of charging the body to be charged at a potential corresponding to a voltage applied to the body to be charged even when the voltage is lower than a discharging threshold value. The injection charging mechanism allows no product to be formed by discharging and is not accompanied by the adverse influence due to a product formed by discharging without any generation of ions.
However, a charging property of the injection charging mechanism is largely dependent on a contact condition of the contacting charging member which is brought into contact with the body to be charged. Accordingly, the injection charging mechanism requires composing the contacting charging member so that it is denser, rotated at a velocity having a larger difference from a rotating velocity of the body to be charged and brought into contact with the body to be charged at a higher frequency.
The inventors have therefore proposed novel charging methods which inject electric charges by way of electrical conductive particles in U.S. patent application Ser. Nos. 35,108, 35,109 and 35,022.
U.S. patent application Ser. No. 35,022 in particular proposes a method which replenishes the electrical conductive particles from a developing apparatus to make up for insufficiency of the electrical conductive particles.
When the electrical conductive particles are consumed at a high rate and it is desired to supply the electrical conductive particles at a high rate from the developing apparatus, it is sufficient to mix the electrical conductive particles at a higher ratio with a toner, but the higher ratio of the electrical conductive particles poses a problem that it makes triboelectricity of the toner unstable and causes defective image transfer.
Furthermore, the electrical conductive particles are supplied at a low rate and apt to be insufficient in a charging portion of the developing apparatus such as a contacting developing apparatus in particular which produces electric fields having low electric field intensities for development as in the contacting developing method.